Overview
This lecture explains key measures of health related to body weight: Body Mass Index (BMI), body fat content, and fat distribution, including their calculation, significance, and limitations.
Body Weight, Overweight, and Obesity
- Overweight means more weight than normal for height, possibly from fat, muscle, or water.
- Obesity is specifically excess body fat, a stronger indicator of disease risk.
- Ideal body weight depends on frame size, sex, muscle, bone density, age, and height.
Body Mass Index (BMI)
- BMI is calculated using height and weight to estimate body fatness.
- BMI categories: underweight (<18.5), normal (18.5–24.9), overweight (25–29.9), obese (≥30).
- High BMI is linked to increased risk of heart disease, diabetes, and chronic conditions.
- BMI risk thresholds differ among population groups (e.g., Asians at lower BMIs).
Calculating and Interpreting BMI
- BMI formula (lb/in): [weight (lb) × 703] / [height (in)]².
- BMI formula (kg/m): weight (kg) / [height (m)]².
- BMI calculators are available from the CDC and NIH.
Limitations of BMI
- BMI does not distinguish between fat and muscle.
- Muscular individuals may be misclassified as overweight or obese.
- Older adults with low bone mass may have deceptively lower BMI.
- BMI is a population tool, not a diagnostic of individual health.
Body Fat Content
- Optimal fat content: females 20–30% body weight; males 12–20%.
- Fat content can be measured by:
- Skin-fold test: calipers measure skin thickness at various sites.
- Underwater weighing: compares weight in and out of water.
- Bioelectric Impedance Analysis (BIA): measures electrical flow through body.
- Dual-energy X-ray absorptiometry (DEXA): uses X-rays to estimate fat, bone, and muscle.
Fat Distribution and Health Risk
- Visceral fat (abdominal) is a stronger disease predictor than total fat.
- Waist circumference: >40 in (men) and >35 in (women) indicates higher risk.
- Waist-to-hip ratio (WHR): calculated as waist circumference divided by hip circumference.
- Apple-shaped bodies (central fat) have higher disease risk than pear-shaped (hip fat).
- WHO defines abdominal obesity as WHR >0.90 (males), >0.85 (females).
Key Terms & Definitions
- BMI (Body Mass Index) — A number calculated from height and weight to estimate body fat.
- Visceral Fat — Fat stored in the abdominal cavity, linked to higher health risks.
- Waist-to-Hip Ratio (WHR) — A measure of fat distribution, calculated by dividing waist by hip circumference.
- Skin-fold Test — A method using calipers to estimate body fat by measuring skin thickness.
- Bioelectric Impedance Analysis (BIA) — Technique estimating body fat by electrical current speed.
- Dual-energy X-ray absorptiometry (DEXA) — Imaging technique for assessing bone density and body fat.
Action Items / Next Steps
- Calculate your own BMI and waist-to-hip ratio using provided formulas or online tools.
- Review any interactive learning activities in the online textbook version.